1. Field of the Invention
The present invention relates to a vibration insulating device to be used as an elastic bushing, an engine mount, a center supporting device for a drive shaft (for example, of two-piece type) or the like, and more particularly to an improvement in such a vibration insulating device to effectively insulate even high frequency vibration.
2. Description of the Prior Art
In connection with vibration insulating devices for insulating vibration to be transmistted from one member to another member of a vibration system, a device of the fluid-filled type has been proposed as disclosed in Japanese Patent (First) Publication No. 48-36151, which is configured as follows: An outer cylindrical member is coaxially disposed around an inner cylindrical member. An elastic member made of rubber or other elastomeric material is interposed between the outer and inner cylindrical members to interconnect them. The elastic member is formed at its axial central section with two fluid chambers which are located opposite to one another in the diametrical direction with respect to the inner cylindrical member. The two fluid chambers are communicated with one another through an orifice passage formed in the elastic member. The two fluid chambers are filled with a fluid having a predetermined viscosity.
With such a vibration insulating device, when relatively low frequency vibration is transmitted to the outer or inner cylindrical member, one of the two fluid chambers deforms in a direction to contract while the other fluid chamber deforms in a direction to expand, so that the fluid in the former fluid chamber flows through the orifice passage to the latter fluid chamber. This provides effective vibration insulation or damping due to flow resistance of the fluid flowing through the orifice passage and due to viscosity resistance of the fluid itself.
However, in the event that vibration of high frequency (higher than a predetermined level, for example, 50 Hz) is transmitted to one of the outer and inner cylindrical members, the fluid is unavoidably prevented from flowing through the orifice passage so that the orifice passage is put into a block condition, thereby considerably raising the dynamic spring constant of the vibration insulating device upon receiving the high frequency vibration.